libffi: update to 3.4.7

[bgilbert]

No description provided.

[bgilbert]

Alpine x86_64 (new failures):

276/718 libffi:bhaible / test-call -O2 -DDGTEST=20         FAIL            0.09s   killed by signal 6 SIGABRT
381/718 libffi:call / promotion -O2                        FAIL            0.11s   killed by signal 6 SIGABRT
589/718 libffi:closures / cls_multi_uchar -O2              FAIL            0.12s   killed by signal 6 SIGABRT
593/718 libffi:closures / cls_multi_ushort -O2             FAIL            0.15s   killed by signal 6 SIGABRT
594/718 libffi:closures / cls_multi_ushortchar -O2         FAIL            0.14s   killed by signal 6 SIGABRT

Tons of Alpine riscv64 failures.

MSVC clang-cl (unchanged vs. 3.4.6):

331/331 libffi:closures / unwindtest_ffi_call         TIMEOUT        30.05s   exit status 1

macOS arm64 (unchanged vs. 3.4.6):

475/714 libffi:closures / closure_loc_fn0 -O0              FAIL            0.01s   killed by signal 6 SIGABRT
476/714 libffi:closures / closure_loc_fn0 -O2              FAIL            0.01s   killed by signal 6 SIGABRT

macOS x86_64 (new failures):

261/718 libffi:bhaible / test-call -O2 -DDGTEST=20         FAIL            0.05s   killed by signal 6 SIGABRT
365/718 libffi:call / promotion -O2                        FAIL            0.18s   killed by signal 6 SIGABRT
580/718 libffi:closures / cls_multi_ushortchar -O2         FAIL            0.04s   killed by signal 6 SIGABRT

[bgilbert]

Looks like the new failures are a regression with x86_64 clang: libffi/libffi#879

Marking draft pending a new release.